A downconverter and upconverter are provided which can obtain a sufficient image rejection ratio in a low-Intermediate Frequency (IF) scheme while reducing power consumption and can suppress Error Vector Magnitude (EVM)-related degradation in a zero-IF scheme. A complex-coefficient transversal filter rejects one side of a positive or negative frequency, and converts a Radio Frequency (RF) signal to a complex RF signal configured by real and imaginary parts. A local oscillator outputs a real local signal with a set frequency. A half-complex mixer, connected to the complex-coefficient transversal filter and the local oscillator, performs a frequency conversion process by multiplying the complex RF signal output from the complex-coefficient transversal filter and the real local signal output from the local oscillator, and outputs a complex signal of a frequency separated by the set frequency from a frequency of the RF signal.